I really hope I'm not going to regret writing this later, but right now everything seems to be okay.

After 30-something hours or running MemTest and multiple sessions with the hair dryer and knocking on the case, PSU and mobo, I decided to reinstall Windows.

I formatted the boot partition, wiping all traces of the past WIndows install. So far I've installed only SageTV, TweakUI and Service Pack 3 along with the drivers for the mobo and capture cards. The system has been up since last night without any signs of issues, recording and playing back for pretty much the entire time.

Fingers crossed.